Water often contains inorganic salts, in particular those of calcium and/or barium which may be of sparing solubility in water and tend to deposit on the inside walls of pipes, and in valves and tanks and on heat exchangers as scale. If scale is allowed to build up in a system, it causes increased pressure losses, reduction in flow rate in pipes or heat exchange capacity and ultimately pipe blockage. Scaling of equipment may arise, for example, in the petrochemical industry, in power generation, and in paper pulp manufacture.
In the production of hydrocarbons from subterranean formations the deposition of scale such as barium or strontium sulphate, calcium carbonate, calcium sulphate or calcium fluoride on surfaces and production equipment is a major production problem. Scale build-up reduces well productivity and shortens the lifetime of production equipment.
In paper pulp manufacture, calcium and barium salts are eluted from wood pulp into the process water. Both aluminium sulphate and sulphuric acid are used in paper making processes and sulphate ions combine with calcium and barium ions to form barium sulphate and calcium sulphate respectively, which are sparingly soluble in water and tend to deposit as scale on surfaces of the processing equipment, including rollers.
Traditionally scaling problems have been overcome by addition of scale inhibitors, which are organic compounds which complex at least some of the metals. But these are expensive and contaminate the water.
According to “Inhibition of calcium sulfate scale by a fluidised bed”, J A M Meijer (Section 2.2.), physical scale prevention methods are known. All physical methods only aim at the prevention of scale deposition on the walls of the system, while crystallisation in the bulk of the solution is allowed and sometimes even stimulated. The most effective physical method is said to be the seeding technique. This technique is based on the addition of seed crystals having a large total surface area, which are able to compete with the walls of the system in consuming the supersaturation. The seeds mostly consist of the same material as the mineral scalant, but other materials are also permitted as long as their surface is favourable for crystallisation. In those cases where the mineral scale is able to crystallise in various modifications the seeding technique yields the best results if the better soluble, faster growing modification is allowed to deposit on the seeds. This condition is inverted in the chemical methods, where the slower growing modification is desired.
An example of the seeding technique is provided in U.S. Pat. No. 3,891,394 which relates to a method and apparatus for the reduction of scale formation in fluid handling equipment, particularly in the tubing and hardware employed in pumping equipment (for example, in the petroleum industry). The apparatus is in the form of a specially shaped hollow core through which the pumped fluid flows, an appreciable part of the mineral content of the fluid being thereby caused to enter a crystalline form while remaining suspended in the fluid to be carried through the pump. The core is fabricated from a special formula of a number of metals by means of a process which encourages the formation of a large number of alloys. The specific alloys incorporated in the core material are chosen in a deliberate attempt to simulate the crystal shapes of the important minerals contained in the fluids to be handled by the pump in which the crystal generator is to be installed. As the fluid enters the crystal generator there is a sudden pressure drop accompanied by a sudden increase in flow velocity and a high degree of turbulence. The contact of the dissolved mineral molecules with the alloy crystal encourages the initial formation of the mineral crystal with the alloy crystal having the effect of a “seed” crystal. The abrupt disturbance afforded by the sudden drop in pressure with the accompanying increase in velocity and the turbulence of the fluid within the core also aids the initial crystal formation. As the initial crystals are formed on the alloy surfaces they are immediately washed away by the violent action of the fluid before additional crystals may grow around the initial crystal and before such a growing crystal can attach itself permanently to the alloy surface. As the initial crystal is torn away from the alloy surface the turbulence of the fluid produces a high degree of exposure of the crystal with additional mineral molecules of the same type so that the freed and suspended initial crystal rapidly grows as it passes though the core and upward into the tubing. The crystals leave the crystal generator in the form of a suspended “snow” which continues to serve as a constant supply of “seed” crystals around which further crystallisation can occur as the fluid flows upward to surface equipment. U.S. Pat. No. 3,891,394 is silent regarding the size of the generated “seed” crystals.
A further example of the seeding technique is given in EP-A-0916622 which relates to a process for preventing scale formation in a paper-making process which comprises adding crystals as seed for crystallisation which have the same or similar form to a scale substance formed in the paper-making process, thus encouraging the scale-forming components to precipitate onto the crystal surfaces. The scales may be calcium sulfate, barium sulfate or calcium oxalate. It is preferable that the seed crystal has an average diameter of 0.05 to 100 μm, more preferably 5 to 50 μm.
Another physical method is said to be ultrasonic treatment (see “Inhibition of calcium sulfate scale by a fluidised bed”, J A M Meijer, Section 2.2), which causes an erosion of the developing scale layer on the wall. The so-formed particles can further act as seed crystals. Since this method is said to be only feasible for small systems, its application is limited. Ultrasonic energy has also been applied to heat exchangers to remove scale and in well bores of oil wells (see e.g. I. A. Beresnev and P. A. Johnson, Geophysics, Vol. 59 No. 6, June 1994 pp 1000-1017).